1. Field of the Invention
The present invention relates to a label sheet for an electrophotographic process that is used for indirect dry electrophotographic copiers and printers and an image forming method using it.
2. Description of the Related Art
With the development of color electrophotographic copiers and printers and digital ones, investigations have been made to increase the image quality and the processing speed of the electrophotographic process. In particular, in full-color electrophotographic copiers and printers, to increase the image quality and the processing speed, great progress has been made in the digitization of the image input/output technologies, whereby the image input method, the method for processing an input image, the development method, the transfer method, the fixing method, etc. have been improved very much. Further, developers and photoreceptor image forming materials have been improved to enable digital color printing that is high in resolution and color formation performance.
On the other hand, with the increase in the image quality of full-color copiers and printers, not only woodfree paper but also other various kinds of medium such as coated paper having a heavy basis weight, cast coated paper, plastic materials, and seal materials have come to be used as printing media. In particular, for seal and label materials, various uses such as labels and photographic seals have been proposed. However, woodfree-paper-based label paper cannot provide high-quality images because heated and melted toner tends to soak into it and, as a result, images appear tough and their gloss is not uniform.
To solve the above problem, as disclosed in Japanese Patent No. 2772570, an attempt to increase the image quality was made in which film having a very smooth surface is used as a label body. However, since coated paper is used as a separator, blisters may occur in the separator depending on the printing environment or after fixing the separator absorbs water and expands to form a large curl.
One method for solving this problem would be to use film for both of the label body and the separator. However, in this case, the entire label sheet becomes an insulator and hence a transfer failure tends to occur in toner transfer regions. To improve the toner transfer performance of label sheets made of film, JP-A Nos. 2000-250248, 6-337537, 6-332222, and 6-95420 proposed controlling the surface resistivity of label sheets by a surfactant. However, where a surfactant is used, the level of fixing of toner to the label sheet surface is low and hence toner is prone to peel off the film.
As disclosed in JP-A Nos. 6-301231, 6-301230, and 6-301229, it was attempted to attain satisfactory performance in both of the toner fixing and the toner transfer by controlling the surface resistivity of label sheets using a metal oxide. Although the toner fixing performance can be improved, a toner transfer failure tends to occur if the balance between the sure resistivity values of the front and rear surfaces of a label sheet is improper, that is, a case that the surface resistivity of the toner accepting surface is lower than the back surface and a case that both of the front and back surfaces have high surface resistivity. Even in a case that both of the font and back surfaces have low surface resistivity, a transfer failure tends to occur in a low-temperature, low-humidity environment. The above combinations of surface resistivity values of the front and back surfaces cannot provide satisfactory performance in both of the toner fixing and the toner transfer.
To eliminate gloss unevenness in the surface of a label sheet, Japanese Patent No. 2659792 and JP-A Nos. 2000-98647, 7-271079, 7-248636, 7-9624, and 6-332222 propose using a toner accepting layer for accepting a toner. However, since Tg (glass transition temperature) of a resin that forms the toner accepting layer is low, blocking may occur. Where this technique is applied to electrophotographic copiers and printers having an oilless fuser that is the mainstream nowadays, since the toner accepting layer tends to adhere to the roll of the oilless fuser, a label sheet may wind around the fixing roll to destroy it.
As described above, existing label sheets for electrophotographic process cannot satisfy all the requirements that satisfactory performance be attained in both of the toner transfer and the toner fixing, that winding of a label sheet around an oilless fuser be prevented, and that produced images have uniform gloss.